(i) Field of the Invention
This invention concerns a heat resistant pad for use with a cooling table or a spacer in the extrusion forming processes in the fields of non-ferrous metal industries such as for aluminium or in heat treatment processes in ceramic industries or the like.
More specifically, it relates to a felt-structured heat resistant pad for use with rear facilities of an aluminium extrusion pressing machine, such as a canister, initial table, run-out table, lift arm and cooling table.
(ii) Description of the Prior Art
Rear facilities of an aluminium extrusion pressing machine are used for receiving or transporting the extruded products at high temperature (550.degree.-600.degree. C.) shaped through an extrusion die. It is required that the heat resistant pad for use with such facilities satisfy the following criteria:
(1) capability of withstanding high temperatures of about 600.degree. C.,
(2) not scratching the surfce of the extruded products,
(3) adequate cushioning property, with no grooved traces left by the extruded products,
(4) adequate friction coefficient so as not to allow excess slip of the extruded products,
(5) low heat conductivity,
(6) low hydroscopic property,
(7) no generation of decomposed gases nor deposition by melting upon contact with the extruded product, and
(8) sufficient wear resistance and impact resistance under high temperature conditions.
However, since conventional heat resistant pads are made of plates of materials such as synthetic or natural carbon, woven fabrics of asbestos or glass fibers and plates of teflon resin, they suffer from the following disadvantages:
(a) Heat Resistant Pads Made of Plates of Synthetic or Natural Carbon
Since they lack in wear resistance and tend to have grooved traces at the surface thereof formed by the extruded products, the succeeding extruded products are often caught in the traces and damaged. Further, since they have high heat conductivity, portions of the extruded products undergoing cooling will be at different temperatures depending on whether they are in contact with the surfaces of the pad, which temperature differences result in an altering of the crystal structure of aluminium. Consequently, cooling produces remarkable dimensional errors, formation of pits in the rapidly cooled portions of the extruded products depending on their cross sectional profile, or results in clouding and so-called black spots or white spots in the subsequent surface treatment or like additional steps. Furthermore, since they have an excessively low frictional coefficient, the extruded products can not be transported due to excess slip when such materials are used in a lift arm or a cooling table.
(b) Heat Resistant Pads Made of Woven Fabrics of Asbestos or Glass Fibers
While they are usually woven into cloth and affixed to a metallic core in use, they lack in flexibility and tend to damage the extruded products. Further, they are susceptible to injury from impact applied from the edges of the extruded products, and the pad body inevitably undergoes attrition to form powdery dusts which worsen the working atmosphere.
(c) Heat Resistant Pads Made of Teflon Resin
Just like in (a) above, poor wear resistance often leads to injury by the extruded products and insufficient frictional coefficient causes trouble in the transportation of the extruded products. Furthermore, the resin may possibly melt and to deposit on the extruded products.